22205-99-8Relevant articles and documents
Bimolecular hydrogen abstraction from phenols by aromatic ketone triplets
Lathioor, Edward C.,Leigh, William J.
, p. 291 - 300 (2008/02/05)
Absolute rate constants for hydrogen abstraction from 4-methylphenol (para-cresol) by the lowest triplet states of 24 aromatic ketones have been determined in acetonitrile solution at 23°C, and the results combined with previously reported data for roughly a dozen other compounds under identical conditions. The ketones studied include various ring-substituted benzophenones and acetophenones, α,α,α-trifluoroacetophenone and its 4-methoxy analog, 2-benzoylthiophene, 2-acetonaphthone, and various other polycyclic aromatic ketones such as fluorenone, xanthone and thioxanthone, and encompass n,π*, π,π*(CT) and arenoid π,π* lowest triplets with (triplet) reduction potentials (Ered*) varying from about -10 to -38 kcal mol-1. The 4-methylphenoxyl radical is observed as the product of triplet quenching in almost every case, along with the corresponding hemipinacol radical in most instances. Hammett plots for the acetophenones and benzophenones are quite different, but plots of log log kQ vs Ered* reveal a common behavior for most of the compounds studied. The results are consistent with reaction via two mechanisms: a simple electron-transfer mechanism, which applies to the n,π* triplet ketones and those π,π* triplets that possess particularly low reduction potentials, and a coupled electron-/proton-transfer mechanism involving the intermediacy of a hydrogen-bonded exciplex, which applies to the π,π* ketone triplets. Ketones with lowest charge-transfer π,π* states exhibit rate constants that vary only slightly with triplet reduction potential over the full range investigated; this is due to the compensating effect of substituents on triplet state basicity and reduction potential, which both play a role in quenching by the hydrogen-bonded exciplex mechanism. Ketones with arenoid π,π* states exhibit the fall-off in rate constant that is typical of photoinduced electron transfer reactions, but it occurs at a much higher potential than would be normally expected due to the effects of hydrogen-bonding on the rate of electron-transfer within the exciplex.
The reactivity of ketyl and alkyl radicals in reactions with carbonyl compounds
Denisov
, p. 2110 - 2116 (2007/10/03)
A parabolic model of bimolecular radical reactions was used for analysis of the hydrogen transfer reactions of ketyl radicals: >C+OH + R1COR2 → >C=O + R1R2C+OH. The parameters describing the reactivity of the reagents were calculated from the experimental data. The parameters that characterize the reactions of ketyl and alkyl radicals as hydrogen donors with olefins and with carbonyl compounds were obtained: >C+OH + R1CH=CH2 → >C=O + R1C+ HCH3; >R1CH=CH2 + R2C+HCH2R3 → R2C+HCH3 + R2CH=CHR3. These parameters were used to calculate the activation energies of these transformations. The kinetic parameters of reactions of hydrogen abstraction by free radicals and molecules (aldehydes, ketones, and quinones) from the C-H and O-H bonds were compared.
Reactivity of HO2. Radicals in Alcohols
Keszler, Agnes,Irinyi, Gyoergy,Heberger, Karoly,Gal, Dezso
, p. 175 - 179 (2007/10/02)
Photoinitiated liquid phase oxidation of 1-phenyl ethanol and benzyl alcohol have been investigated in the temperature range of 50-70 deg C using the intermittent illumination (rotating sector) technique.The rate of oxidation was followed by gas chromatography while the rate of initiation was determined by the inhibitor method.Results were explained by assuming HO2. radicals to be the main chain carriers, supported by comparison of the results with literature data. - The termination rate constant of HO2. was calculated from the average lifetime of radicals in both alcohols: 2kt=9.91*1012 exp(-29.2 kJ*mol-1/RT) in 1-phenyl ethanol and 2kt=8.98*1012 exp(-31.3 kJ*mol-1/RT) in benzyl alcohol. (in l*mol-1*s-1, the uncertainties given by the covariance matrix, see in the text.) - The rate constants for hydrogen abstraction by HO2. from the substrate molecules have also been determined in the case of both alcohols: kp=4.61*108 exp(-52.0 kJ*mol-1/RT) in 1-phenyl ethanol, kp=1.04*109 exp(-54.3 kJ*mol-1/RT) in benzyl alcohol, (in l*mol-1*s-1, uncertainties see in the text.) Keywords: Chemical Kinetics/ Elementary Reactions / Photochemistry / Radicals
